Yes it is similar to vertex groups, but it would be more like a face group. Since you actually assign the face to the material slot. Technically speaking each polygon is made up of a bunch of parts. Like a face and the face’s number(Instance), which material slot the face is assigned to. It also has the number of edges that make up that face and their numbers. Then there is the vertices and there numbers along with each one’s position in the meshes local space. Each vertex is also assigned a color. This is what vertex painting is used for. There is also information on face corners, but I believe that is stored at the object level not the individual polygon. Martian would know better than me on this.
